NMR local coil for brain imaging

Abstract

A local RF and gradient coil for acquiring images of the human brain using fast NMR pulse sequences includes an end capped RF bird cage coil surrounded by a 3 axis gradient coil assembly. An RF shield is disposed between the RF coil and the gradient coils and it is divided into separate segments to reduce eddy currents induced by the changing gradient fields.

We claim: 
     
       1. A local RF and gradient field coil for acquiring NMR data, which comprises: an end capped RF bird cage coil shaped to fit around the cranium of a human subject;   a cylindrical RF shield disposed around and fastened to the end capped RF bird cage coil; and   a cylindrical gradient coil assembly disposed around and fastened to the cylindrical RF shield, and including coils for producing three orthogonal magnetic field gradients.   
     
     
       2. The local RF and gradient field coil as recited in claim 1 in which the end capped RF bird cage coil includes a set of conductive linear elements Joined at one of their ends by a conductive end cap and Joined at the other of their ends by an elliptical shaped conductive end ring. 
     
     
       3. The local RF and gradient field coil as recited in claim 2 in which the conductive end cap includes a set of wedge-shaped conductive elements capacitively coupled together and the elliptical shaped conductive end ring includes a capacitor element which open circuits the conductive end ring. 
     
     
       4. The local RF and gradient field coil as recited in claim 1 in which the cylindrical RF shield is formed by a set of separate capacitively coupled conductive segments. 
     
     
       5. The local RF and gradient field coil as recited in claim 1 in which the cylindrical RF shield includes a circular cylindrical substrate having a first set of conductive segments formed on its outer surface and a second set of conductive segments formed on its inner surface. 
     
     
       6. The local RF and gradient field coil as recited in claim 5 in which the first set of conductive segments are aligned along the axial direction of the circular cylindrical substrate and the second set of conductive segments overlap gaps formed between the first set of conductive segments. 
     
     
       7. The local RF and gradient field coil as recited in claim 1 in which each of the three gradient field coils is formed from copper wire wound in a circular cylindrical shape. 
     
     
       8. The local RF and gradient field coil as recited in claim 1 in which the end capped RF bird cage coil may be removed from the surrounding cylindrical RF shield and cylindrical gradient coil assembly. 
     
     
       9. The local RF and gradient field coil as recited in claim 8 in which the end capped RF bird cage coil defines a substantially elliptical cylindrical volume for receiving the cranium of the human subject, and the RF coil assembly defines a circular cylindrical volume defined by the cylindrical RF shield. 
     
     
       10. The local RF and gradient field coil as recited in claim 1 in which the end capped RF bird cage coil includes a set of conductive linear elements disposed around the cranium of the human subject and Joined at one of their ends by a conductive end cap which is disposed against the top of the cranium of the human subject. 
     
     
       11. The local RF and gradient field coil as recited in claim 10 in which a capacitor element is formed in each conductive linear element and a pair of RF cables connect to the end capped RF bird cage coil by connection to two conductive linear elements across their respective capacitor elements. 
     
     
       12. The local RF and gradient field coil as recited in claim 11 in which each of said RF cables makes its connection through a capacitor. 
     
     
       13. The local RF and gradient field coil as recited in claim 11 in which the RF cables make their connections to two respective linear elements that are disposed in quadrature about the cranium of the human subject and are located on diagonal axes that pass through the center of the cranium of the human subject. 
     
     
       14. A local RF and gradient field coil for acquiring NMR data, which comprises: an RF coil shaped to fit around a region for receiving the cranium of a human subject;   a cylindrical RF shield disposed around the RF coil; and   a cylindrical gradient coil assembly disposed around the cylindrical RF shield and including three separate coils for producing three respective orthogonal magnetic field gradients within the cranium of a human subject positioned within the RF coil.   
     
     
       15. The local RF and gradient field coil as recited in claim 14, in which each of the three separate coils in the cylindrical gradient coil assembly is formed from copper wire. 
     
     
       16. The local RF and gradient field coil as recited in claim 14, in which each of the three separate coils in the cylindrical gradient coil assembly is wound in a circular cylindrical shape. 
     
     
       17. The local RF and gradient field coil as recited in claim 16 in which each of the three separate coils are formed of copper wire disposed in grooves formed in a circular cylindrical insulating substrate. 
     
     
       18. The local RF and gradient field coil as recited in claim 17 in which the circular cylindrical insulating substrate is formed as three layers and each of the three separate coils is disposed in grooves formed in each of the three layers. 
     
     
       19. The local RF and gradient field coil as recited in claim 14 in which a first one of the three separate coils is formed by a set of four substantially identical coil patterns disposed in a cylindrical shape and positioned symmetrically about an isocenter location within the region for receiving the cranium of a human subject. 
     
     
       20. The local RF and gradient field coil as recited in claim 19 in which a second one of the three separate coils is formed by a set of four substantially identical coil patterns disposed in a cylindrical shape and positioned symmetrically about the isocenter. 
     
     
       21. The local RF and gradient field coil as recited in claim 20 in which a third one of the three separate coils is formed as a pair of circular cylindrical coils disposed anti-symmetrically about the isocenter. 
     
     
       22. The local RF and gradient field coil as recited in claim 14 in which the cylindrical gradient coil assembly and the cylindrical RF shield have substantially the same axial length, and the RF coil is substantially shorter than said axial length and is centered within the cylindrical volume formed by the cylindrical gradient coil assembly. 
     
     
       23. The local RF and gradient field coil as recited in claim 22 in which the RF coil is an end capped bird cage coil and is located within said cylindrical volume such that a gradient field isocenter is located within a central region of the human subject's brain when the cranium is positioned against the end cap. 
     
     
       24. The local RF and gradient field coil as recited in claim 14 in which the cylindrical gradient coil assembly includes: a cylindrical insulating substrate;   a first of said three separate coils is formed from conductive wire and disposed on the cylindrical insulating substrate in a shape which produces a magnetic field along a central axis of said region with a first gradient along said central axis;   a second of said three separate coils is formed from conductive wire and disposed on the cylindrical insulating substrate in a shape which produces a magnetic field along said central axis of said volume with a second gradient perpendicular to said central axis; and   a third of said three separate coils is formed from conductive wire and disposed on the cylindrical insulating substrate in a shape which produces a magnetic field along said central axis of said volume with a third gradient perpendicular to said first and second gradients;   wherein the shapes of the three gradient coils are such that a net force imposed on the local RF and gradient field coil as a result of its interaction with a polarizing magnetic field is substantially zero.   
     
     
       25. The local RF and gradient field coil as recited in claim 24 which includes means for supporting the local RF and gradient field coil on a table supporting the human subject and for aligning the local RF and gradient field coil to receive the head of the human subject. 
     
     
       26. The local RF and gradient field coil as recited in claim 14 in which the cylindrical gradient coil assembly includes: a substrate formed about a z-axis to define a volume for receiving the cylindrical RF shield; and said three separate coils comprise:   a z-axis gradient coil disposed on the substrate for producing a magnetic field gradient along the z-axis;   an x-axis gradient coil disposed on the substrate for producing a magnetic field gradient along an x-axis which is perpendicular to the z-axis; and   a y-axis gradient coil disposed on the substrate for producing a magnetic field gradient along the y-axis which is perpendicular to the z-axis;   wherein the three gradient coils have shapes such that: a) the three perpendicular magnetic field gradients are uniform throughout an entire region occupied by a brain of the human subject received in said volume;   b) all three gradient coils are symmetric about the z-axis;   c) the x-axis and y-axis gradient coils are symmetric about the x-axis and the y-axis;   d) the z-axis gradient coil is antisymmetric about the x-axis and the y-axis; and   e) the length of the three gradient coils along the z-axis does not exceed twice a distance from the center of the subject's brain to the subject's shoulders.     
     
     
       27. A local RF coil for acquiring NMR data from the head of human subject, which comprises: a conductive end cap comprised of a set of capacitively coupled conductive segments;   a set of conductive linear elements each having one end connected to the conductive end cap and the conductive linear elements extending parallel therefrom to define a volume which receives the head of the human subject; and a conductive end ring connected to the other end of each conductive linear element.   
     
     
       28. The local RF coil as recited in claim 27 in which the set of conductive linear elements connect to the conductive end cap in an elliptical pattern, the conductive end ring is elliptical shaped, and the volume is elliptical cylindrical in shape.